1. Field of the Invention
The present invention relates to digital TV receivers, and more particularly, to a device for recovering a carrier in a VSB system digital TV receiver.
2. Background of the Related Art
The VSB (vestigial sideband) system, employed as Korean and the USA digital TV (DTV) broadcasting standards, is designed to transmit a broadcasting signal by using a frequency allocated for the present analog TV broadcasting. However, for minimizing an influence to the present analog TV broadcasting, the DTV signal is transmitted at a very low intensity compared to the analog TV signal. Of course, the standards are set such that there is no problem in reception of the DTV signal even if the intensity of the signal is low by employing different coding system in the DTV signal and channel equalizers for reducing an influence from noise. However, if a situation of the transmission channel is very poor, the reception of the signal is very poor. In general, since the DTV transmission system has an advantage in that a noise occurred on a transmission channel is removed perfectly at reception of the broadcasting signal, to enable to watch a picture having no noise at all, while the DTV transmission system has a disadvantage in that the picture can not be watched at all if the transmission signal is not recovered fully, it is required that the DTV receiver receives whole signal whatever poor transmission channel the signal has passed.
FIG. 1 illustrates a block diagram of a related art VSB system DTV receiver, wherein, upon reception of a RE (Radio Frequency) signal modulated in a VSB system through an antenna 101, after selecting a particular channel frequency the user desires, a tuner 102 converts the RF band VSB signal carried on the channel frequency into an IF band (Intermediate Frequency band) signal (in general 44 MHz or, in the case of analog TV broadcasting, 43.75 MHz is used widely), and filters out other channel signals, appropriately.
The signal from the tuner 102, which converts a spectrum of channels into an IF pass band signal, passes a SAW (Surface Acoustic Wave) filter 103 employed for removing neighboring channel signals, and the noise signal.
In this instance, since the digital broadcasting signal has all information within a 6 MHz zone centered on, for an example, the IF of 44 MHz the SAW filter 103 removes other zones from the output of the tuner 102, only leaving the 6 MHz zone having the information, and provides to an IF amplifier 104.
The IF amplifier 104 multiplies a gain calculated beforehand to the output from the SAW filter 103 for making an amplitude of the signal from an A/D (analog/digital) converter after the IF amplifier 104 the same always. Accordingly, the A/D converter 105 receives and digitizes signals of the same amplitude from the IF amplifier 104. The passband signal digitized at the A/D converter 105 is transited to a baseband signal at the carrier recoverer 106, and provided to a DC (direct current) remover 107. In this instance, the carrier used in recovering the carrier at the carrier recoverer 106 is turned to a DC component having 0 Hz frequency.
That is, the DC component is forcibly inserted into a transmission signal at a transmitter for the carrier to recoverer to recover the carrier. Therefore, after the carrier recovery is done, the DC component inserted at the transmitter is required no more. According to this, the DC remover 107 detects and removes the DC component from the baseband signal from the carrier recoverer 106.
The baseband digital signal having the DC component removed therefrom is provided both to a synchronizer 108 and a channel equalizer 109.
The most remarkable features of the VSB transmission system suggested by the Grand Alliance (GA) compared to other DTV transmission system are a pilot signal, a data segment synchronizing signal, and a field synchronizing signal. The signals are inserted at the transmitter before transmission for improving characteristics of carrier recovery and timing recovery.
Accordingly, the synchronizer 108 recovers the data segment synchronizing signal, and the field synchronizing signal from the signal inserted at the time of transmission having the DC component removed therefrom. The synchronizing signals obtained thus are provided to the channel equalizer 109, a phase corrector 110, and an FEC (forward error correction) 111.
The channel equalizer 109 removes a linear distortion of an amplitude in the baseband digital signal causing interference between symbols, ghost occurred as the signal is reflected at buildings or mountain, and the like by using the baseband digital signal and the synchronizing signal, and provides to the phase corrector 110.
Referring to FIG. 1, the signal passed through all analog processes is converted into a digital signal at the A/D converter 105 and provided to the carrier recoverer 106. Therefore, all of the digital process blocks after the carrier recoverer 106 can not make regular operation if the carrier recovery is not made at the carrier recoverer 106.
Therefore, the carrier recoverer 106 in the DTV receiver recovers a position of a pilot frequency in a frequency of a transmitted signal, and converts the pilot frequency into a baseband signal.
Presently, as the most general algorithm of the carrier recoverer 106, an FPLL (Frequency Phase Locked Loop) as shown in FIG. 2 is used, which is used widely as the circuit is simple and has good performance. That is, the carrier recoverer 106 of the FPLL demodulates passband I, Q signals from the A/D converter 105 into baseband I, Q signals, and locks frequencies and phases.
Referring to FIG. 2, a real component of a passband signal digitized at the A/D converter 105 is provided both to a delay 201 and a Hilbert transformer 202.
The Hilbert transformer 202 receives, and inverts the real component to 90° to transform the real component into an imaginary component signal, and provides to a complex multiplier 203, and the delay 201 receives and delays a real component of the signal as long as a process time period at the Hilbert transformer 202, and provides to the complex multiplier 203.
For convenience of description, the signal passed through the delay 201 is called as an I channel signal, and the signal passed through the Hilbert transformer 202 is called as a Q channel signal.
The complex multiplier 203 receives a complex carrier having carrier recovery done, i.e., a sine wave and a cosine wave, through an NCO (Numerically Controlled Oscillator) 210, and multiplies to the passband I, Q signals from the A/D converter 105, to transit the passband I, Q signals into baseband I, Q signals.
The baseband I, Q signals are provided to a DC remover 107 and, for recovery of the carrier, the baseband I signal is provided to a first lowpass filter 204, and the baseband Q signal is provided to a second lowpass filter 205.
In this instance, the carrier recoverer 106 requires a signal in the vicinity of frequency having the pilot frequency present therein among the 6 MHz bandwidth of signal. Therefore, the first and second lowpass filters 204 and 205 remove rest of frequency component having a data component therein from the I, Q signals, for preventing a performance of the carrier recoverer from being deteriorated by the data.
The first lowpass filter 204 provides an output to a delay 206. The delay 206 delays the I signal having the data component removed therefrom for a time period and provides to a sign extractor 207. If the I signal of the pilot component from the first lowpass filter 204 fails in conversion into a DC component accurately when the I signal of the pilot component passes through the delay 206, a phase error as much as the failure occurs.
Therefore, the delay 206 receives, and converts a difference of a pilot frequency component of the passband signal and the carrier frequency component of the NCO 210 into a form of a phase error, and provides to a sign extractor 207.
The sign extractor 207 only extracts a sign of the signal from the delay 206, and provides to a multiplier 208. The multiplier 208 multiplies the sign of the I signal and the Q signal having the data component removed therefrom, and provides to a loop filter 209 as a phase error. The loop filter 209 receives, filters, and accumulates the phase error, and provides to the NCO 210, and the NCO 210 generates a complex carrier proportional to an output from the loop filer 209, and provides to the complex multiplier 203.
In this instance, if a frequency of the pilot, a component of a carrier present in a received passband, and a frequency component of a carrier generated at the NCO 210 are identical exactly, the service of the carrier recoverer 106 finishes.
Thus, the pilot frequency, information from the transmitter for carrier recovery, is definite, has little phase jitter that can be taken place in the carrier recovery. However, because the pilot is reduced in a signal received in a channel environment having many reflected waves, such as an environment of a city center, use of the pilot at the carrier recoverer is limited. That is, there can be a case the data component is attenuated depending on a DTV reception channel environment, a case a frequency component in the pilot is attenuated, and, in the worst case, there can be a case the frequency component in the pilot is attenuated fully, such that no pilot component exhibits.
In the case the pilot component is reduced or fails to exhibit, accurate carrier recovery can fails. Moreover, the phase jitter, influenced from noise, becomes heavy following the reduction of the pilot.